1. Field of the Invention
This invention relates to a velocity distribution measurement apparatus, and particularly to a velocity distribution measurement apparatus for obtaining velocity distribution characteristics in an object by projecting a beam of coherent light at the object, using a photosensor to pick up and photoelectrically convert the light scattered by the object, and subjecting it to electronic signal processing.
2. Description of the Prior Art
Conventional means which use coherent light such as a laser beam to measure the velocity of a moving object such as, for example, the velocity of a fluid flowing in a glass tube, or of the blood flow in the blood vessel of an eye fundus or other such living organism, include the laser Doppler velocity meter and the laser speckle velocity meter.
With the laser Doppler velocity meter, the laser beam is focused on the measurement zone of the object and the amount of Doppler frequency shift in the light scattered by the object is detected and used to obtain a measurement of the velocity of any scattering bodies in the zone (see, for example, Applied Optics, Vol.24(1985) page 605 or Vol. 25(1986) page 649). The features of this method are high spatial resolution at the point of measurement and good accuracy.
The laser speckle velocity meter involves directing the laser beam onto the object and detecting a speckle pattern produced in the light scattered by the object, then subjecting the detection signals to autocorrelation or cross-correlation functions to determine the velocity of the scattering bodies ( see, for example, Applied Optics, Vol.23(1984) page 2353 or Vol.25(1986) page 22). The feature of this method is that generally the optical system is easy to handle. Here, "speckle pattern" means the irregular pattern of speckles that appear in scattered light from diffusers such as ground glass, coarse metal surfaces, paper, walls and the like subjected to illumination by coherent light, the speckles being produced by interference between the rays of light scattering from each of the points of the diffuser.
There are methods of electronically measuring velocity distribution in an object in one or two dimensions based on such means. However, there are problems with these. For example, to find velocity distributions using the laser Doppler method involves moving either the entire optical system or the object itself or using frequency shifters and TV camera, and the apparatus has a complex structure (see, for example, Applied Optics, Vol.22(1983) page 2448). The laser Doppler velocity meter is particularly difficult to use, the exacting conditions imposed by the optical system making alignment difficult, while the method involving moving the object itself limits the objects that can be measured. Another problem is that the measurement process can become very lengthy if the number of measurement points is raised in order to increase the amount of spatial information.
Apparatuses employing the laser speckle pattern method, using for example a point type optical detector and calculating velocity by using auto-correlation functions on the detection signals, are inherently suited to velocity measurements in a single measurement zone of an object. With this method too, it would be necessary to move the optical system or the object itself, or it would also be necessary to arrange the optical detectors in parallel, which again leads to structural complexity. Again, of course, the method for moving the object limits the objects that can be measured, and again there is the problem that the measurement process can become very lengthy if the number of measurement points is raised in order to increase the amount of spatial information.
Another method of measuring velocity distribution in an object consists of using a CCD or imaging tube to form an optical detector that functions to pick up the speckle pattern in one or two dimensions, and subjecting the detection signals to autocorrelation, or computing variations in the detection signals ( see, for example, Applied Optics, Vol.26(1987) page 5321). However, because the optical detector used in these methods is a one- or two-dimensional image sensor, the quantum efficiency is low, compared to a point-type detector such as a photomultiplier or the like, and when the reflectivity or transmissivity of the object is low, the S/N ratio of the detection signal makes measurement difficult, while another problem that arises when operations such as cross-correlation functions are used is the increasing time required for the computation, making it difficult to increase the number of measurement points.